Superconducting device having an extremely short superconducting channel formed of extremely thin oxide superconductor film

ABSTRACT

A superconducting device includes a superconducting channel constituted in an oxide superconductor the film deposited on a deposition surface of a substrate. A source electrode and a drain electrode are formed on the oxide superconductor thin film at opposite ends of the superconducting channel, so that a superconducting current can flow through be superconducting channel between the superconductor source electrode and the superconductor drain electrode. A gate electrode is formed through a gate insulator layer on the superconducting channel so as to control the superconducting current flowing through the superconducting channel. The gate electrode is in the form of a thin film and stands upright with respect to the gate insulator layer.

This application is a continuation of application No. 07/785,324, filedOct. 31, 1991, which application is entirely incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the invention

The present invention relates to a superconducting device and a methodfor manufacturing the same, and more specifically to a superconductingdevice including an extremely thin superconducting channel formed ofoxide superconductor material, and a method for manufacturing the same.

2. Description of related art

Typical devices utilizing a superconductor include a so called Josephsondevice, which comprises a pair of superconductors coupled to each otherthrough a tunnel barrier. The Josephson device can realize a high speedswitching. However, the Josephson device is a two-terminal device, andtherefore, requires a complicated circuit in order to realize a logiccircuit.

On the other hand, typical three-terminal devices utilizing asuperconductor include a so called superconducting-base transistor and aso called super-FET (field effect transistor). The superconducting-basetransistor includes an emitter of a superconductor or a normalconductor, a tunnel barrier of an insulator, a base of a superconductor,a semiconductor isolator and a collector of a normal conductor, stackedin the named order. This superconducting-base transistor operates at ahigh speed with a low power consumption, by utilizing high speedelectrons passing through the runnel barrier.

The super-FET includes a semiconductor layer, and a superconductorsource electrode and a superconductor drain electrode which are formedclosely to each other un the semiconductor layer, A portion of thesemiconductor layer between the superconductor source electrode and thesuperconductor drain electrode has a greatly recessed or undercut rearsurface so as to have a reduced thickness. In addition, a gate electrodeis formed through a gate insulating layer on the recessed or undercutrear surface of the portion of the semiconductor layer between thesuperconductor source electrode and the superconductor drain electrode.

A superconducting current flowing through the semiconductor layerportion between the superconductor source electrode and thesuperconductor drain electrode due to a superconducting proximityeffect. is controlled by an applied gate voltage. This super-FET alsooperates at a high speed with a low power consumption.

In addition, in the prior art, there has been proposed a three-terminalsuperconducting device having a channel of a superconductor formedbetween a source electrode and a drain electrode, so that a currentflowing through the superconducting channel is controlled by a voltageapplied to a gate formed above the superconducting channel.

Both of the above mentioned superconducting-base transistor and thesuper-FET have a portion in which a semiconductor layer and asuperconducting layer are stacked to each other. However, it isdifficult to form a stacked structure of the semiconductor layer and thesuperconducting layer formed of an oxide superconductor which hasrecently advanced in study. In addition, even if it is possible to forma stacked structure of the semiconductor layer and the oxidesuperconducting layer, it is difficult to control a boundary between thesemiconductor layer and the oxide superconducting layer. Therefore, asatisfactory operation could not been obtained in these superconductingdevices.

In addition, since the super-FET utilizes the superconducting proximityeffect, the superconductor source electrode and the superconductor drainelectrode have to be located close to each other at a distance which isnot greater than a few times the coherence length of the superconductormaterials of the superconductor source electrode and the superconductordrain electrode. In particular, since an oxide superconductor has ashort coherence length, if the superconductor source electrode and thesuperconductor drain electrode are formed of the oxide superconductormaterial, a distance between the superconductor source electrode and thesuperconductor drain electrode has to be not greater than a few tennanometers. However, it is very difficult to conduit a fine processingsuch as a fine pattern etching so as to ensure the very short separationdistance. Because of this, in the prior art, it has been impossible tomanufacture the super-FET composed of the oxide superconductor materialwith good reproducibility.

Furthermore, it has been confirmed that the conventional three-terminalsuperconducting device having the superconducting channel shows amodulation operation. However, the conventional three-terminalsuperconducting degree having the superconducting channel could notrealize a complete ON/OFF operation, because a carder density is toohigh. In this connection, since an oxide superconductor material has alow carrier density, it is compacted to form a three-terminalsuperconducting device which has a superconducting channel and which canrealize the complete ON/OFF operation, by forming the superconductingchannel, of the oxide superconductor material. In this case, however, athickness of file superconducting channel has to be made on the order offive nanometers. This extremely thin superconducting channel isdifficult to realize.

On the other hand, in order to realize a high speed ON/OFF operation illthe above mentioned superconducting device, it is necessary to shorten agate length. In order to shorten the gate length, the gate electrode hasto have n shape which is short in a direction of a current flowingthrough the superconducting channel, for example, not greater than 100nm. It is practically very difficult to form the gate electrode havingthe above mentioned size on the oxide superconductor by a conventionalfine-working (fine-etching) technique with good reproducibility.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide asuperconducting device and a method for manufacturing the same, whichhave overcome the above mentioned defects of the conventional ones,

Another object of the present invention is to provide a superconductingdevice including a superconducting region constituted of an extremelythin oxide superconductor film, which can be manufactured by usingexisting established processing techniques with a good repeatability.

Still another object of the present invention is to provide an FET typesuperconducting device including an extremely short superconductingchannel composed of an oxide superconductor thin film, and a method formanufacturing the same with a good repeatability by using existingestablished processing techniques.

The above and other objects of the present invention are achieved inaccordance with the present invention by a superconducting deritccomprising a substrate, a superconducting channel constituted in anoxide superconductor thin film deposited on a deposition surface of thesubstrate a source electrode and a drain electrode formed on the oxidesuperconductor thin film at opposite ends of the superconductingchannel, so that a superconducting current can flow through thesuperconducting channel between the superconductor source electrode andthe superconductor drain electrode, and a gate electrode formed througha gate insulator layer on the superconducting channel so as to controlthe superconducting current flowing through the superconducting channel,the gate electrode being in the form of a thin film standing upright togate insulator layer.

In one embodiment, the superconducting channel has a thickness of notgreater than five nanometers, and the oxide superconductor thin film hasan upper planar surface. The gate insulator layer is deposited on theupper planar surface of the oxide superconductor thin film, and aninsulating protection layer is formed on the gate insulator layer andhaving an end surface angled to the gate insulator layer. The gateelectrode is formed of a thin film of a normal conductor deposited onthe end surface of the insulating protection layer.

The above mentioned superconducting device can be formed in accordancewith a method of the present invention, by forming on a substrate anoxide superconductor thin film having a planar upper surface and asuperconducting region having a thickness of not greater than fivenanometers, depositing a gate insulator layer on the planar uppersurface of the oxide superconductor thin film, forming a plurality ofinsulating protection layers on the gate insulator layer in such amanner that a selected one of the insulating protection layers has anend surface positioned above the superconducting region having thethickness of not greater than five nanometers, and forming a gateelectrode in the form of a thin film on the end surface of the selectedinsulating protection layer.

In another embodiment, the gate electrode being in the form of a thinfilm is embedded in the substrate. This gate electrode in the form of athin film and embedded in the substrate can be formed by depositing anoxide superconductor thin film on an insulating substrate having astepped portion or a semiconductor substrate having a stepped portionand coated with an insulating layer, selectively removing the depositedoxide superconductor thin film from the substrate so as to leave thedeposited oxide superconductor thin film on only the side surface of thestepped portion, and filling a material (for example, the same materialas that of the substrate) into a recess formed by the stepped portion.

In still another embodiment, the gate electrode being in the form of athin film is located on the substrate and the source electrode and thedrain electrode are formed of an oxide superconducting source electrodeand an oxide superconducting drain electrode, respectively, which arelocated on the oxide superconductor thin film of the superconductingchannel though aim extremely thin insulator layer. Flocculation occursin the extremely thin insulator layer sandwiched between the oxidesuperconductor thin film of the superconducting channel and each of theoxide superconducting source electrode and the oxide superconductingdrain electrode, so that the superconducting source electrode and thesuperconducting drain electrode are in electrical connection with theoxide superconductor thin film of the superconducting channel.

In this embodiment, the oxide superconductor thin film of thesuperconducting channel is formed of a c-axis oriented oxidesuperconductor crystal layer formed on the substrate, and thesuperconducting source electrode, the superconducting gate electrode andthe superconducting drain electrode are formed of an a-axis orientedoxide superconductor crystal layer formed on the extremely thininsulator layer.

As seen from the above, the superconducting device in accordance withthe present invention includes the superconducting channel formed of theoxide superconductor thin film, the source electrode and the drainelectrode for causing a current to flow through, the superconductingchannel, and the gate electrode for controlling the current flowingthrough the superconducting channel. Here, each of the three electrodesmust be not necessarily constituted of a superconducting electrode.

As mentioned above, in the superconducting device in accordance with thepresent invention, the superconducting channel is constituted of aportion of the oxide superconductor thin film having the planar uppersurface. In order to turn on and off the gate (namely, thesuperconducting channel) by a voltage applied to the gate electrode, thethickness of the superconducting channel in a direction of an electricfield created by the gate electrode must be not greater than fivenanometers. Such an extremely thin oxide superconductor thin film can beformed in a conventional process by precisely controlling the growthspeed and the growth time of the thin film. For this purpose, asputtering can be used. However, since the oxide superconductor crystalhas a multi-layer structure, in which respective constituent elementsare stacked in a layered structure, it is possible to stack a desirednumber of unit cells of oxide superconductor, by using a MBE (molecularbeam epitaxy).

In the superconducting channel-FET having a channel formed of an oxidesuperconductor thin film, only a portion of the oxide superconductorthin film subjected to an electric field given by an applied gatevoltage can flow and block an electric current. Therefore, the channellength is substantially determined by the gate length of the gateelectrode. The gate length is a length of the gate electrode in adirection of a current flowing through the superconducting channel.Namely, in the superconducting device in accordance with the presentinvention, since the extremely thin gate electrode stands upright on thesuperconducting channel through the gate insulator layer, the gatelength of the gate electrode is substantially determined by thethickness of the oxide superconductor thin film of the gate electrode.Preferably, the gate electrode has a thickness of not greater than 100nm. In any case, the thinner the gate electrode (in the form of a thinfilm) becomes, the shorter the gate length becomes. Accordingly, in thesuperconducting device in accordance with the present invention, anextremely short gate length and hence a corresponding extremely shortsuperconducting channel can be realized by this extremely thin gateelectrode, so that the ON/OFF operation can be speeded up.

As mentioned above, only the portion of the oxide superconductor thinfilm superconducting channel layer subjected to an electric field givenby an applied gate voltage constitutes the superconducting channel whichcan flow and block an electric current. Namely, the other portion of theoxide superconductor thin film superconducting channel layer does notcontribute ON/OFF of the current, and therefore, it can be understoodthat, a portion of the oxide superconductor thin film superconductingchannel layer that does not contribute ON/OFF of the current, is aportion of a source electrode or a drain electrode. In thisspecification, therefore, the source electrode should be understood toinclude not only an electrode corresponding to the electrode which iscalled a "source electrode" in the field of a semiconductor MOSFET, butalso a source region which is formed adjacent to and continuous to thesuperconducting channel and on which the source electrode is formed, andthe drain electrode should be understood to include not only anelectrode corresponding to the electrode which is called a "drainelectrode" in the field of the semiconductor MOSFET, but also a drainregion which is formed adjacent to and continuous to the superconductingchannel and on which the drain electrode is formed.

As mentioned above, the thin film of the gate electrode is provided onthe end surface of the insulating protection layer. Therefore, it ispossible to easily form the thin film of the gate electrode on the endsurface of the insulating protection layer, by depositing a conductivematerial onto the end surface of the insulating protection layer.Accordingly, the superconducting device in accordance with the presentinvention can have the above mentioned shortened gate without using thefine-processing technique.

In the superconducting device in accordance with the present invention,the above mentioned extremely thin superconducting channel is realizedas follows:

(1) Components of the substrate are selectively diffused into the oxidesuperconductor layer deposited on the substrate, so that anon-superconducting region is formed in the oxide superconductor thinfilm. The non-superconducting region thus formed in the oxidesuperconductor thin film acts to thin a superconducting portion of theoxide superconductor thin film.

(2) A projection is previously formed on the substrate, and an oxidesuperconductor thin film having a planar upper surface is formed tocover the projection of the substrate. As a result, an extremely thinportion of the oxide superconductor thin film is formed above theprojection of the substrate.

In the former case, constituent element(s) of the substrate can bediffused into the oxide superconductor thin film. Otherwise, a portionof the substrate surface can be previously provided with a layer orisland of a material which diffuses into an oxide superconductor thinfilm in the course of the deposition of the oxide superconductor thinfilm so as to destroy superconductivity in a diffused portion of thedeposited oxide superconductor thin film. In order to cause theconstituent element(s) of the substrate to diffuse into the oxidesuperconductor thin film, energy is locally applied to a position of theoxide superconductor thin film where the superconducting channel is tobe formed, by a focused ion beam, a laser, or the like, so that theconstituent element(s) of a substrate portion under the superconductingchannel forming position of the oxide superconductor thin film is causedto diffuse into the oxide superconductor thin film.

In general, the oxide superconductor has large crystallineinhomogeneity. In particular, the critical current density is larger indirections perpendicular to the c-axis, than in a direction parallel tothe c-axis. Therefore, if a superconducting source electrode and asuperconducting drain electrode have been formed of c-axis orientedoxide superconductor thin films, it has been difficult to cause asuperconducting current to uniformly flow through an extremely thinsuperconducting channel of an oxide superconductor. As mentioned, above,in the superconducting device in accordance with the present invention,since the superconducting source electrode and the superconducting drainelectrode are formed of an a-axis oriented oxide superconductor thinfilm, the main current is allowed to flow within the superconductingsource electrode and the superconducting drain electrode in a directionperpendicular to the substrate. On the other hand, since thesuperconducting channel is formed of a c-axis oriented oxidesuperconductor thin film, the main current is allowed to flow within thesuperconducting channel in a direction parallel to the substrate.Therefore, in each of the superconducting source electrode, thesuperconducting drain electrode and the superconducting channel, themain current is caused to flow in a direction having a large criticalcurrent density of the oxide superconductor crystal.

The c-axis oriented oxide superconductor thin film superconductingchannel can be easily formed by maintaining the substrate at atemperature of about 700° C. when the oxide superconductor thin film isdeposited. On the other hand, the a-axis oriented oxide superconductorthin film superconductor source electrode and superconductor drainelectrode can be easily formed by maintaining the substrate at atemperature of not greater than about 650° C. when the oxidesuperconductor thin film is deposited. In any ease, the oxidesuperconductor thin film can be deposited by a sputtering such as anoff-axis sputtering, a reactive evaporation, an MBE (molecular beamepitaxy), a vacuum evaporation, a CVD (chemical vapor deposition), etc.

In a preferred embodiment of the superconducting device in accordancewith the present invention, the oxide superconductor thin films isformed of a high-Tc (high critical temperature) oxide superconductormaterial. This high-To oxide superconductor material has been studied bymany researchers since the discovery of Bednorz and Muller in 1986, andis said to indicate an oxide superconductor material having a criticaltemperature of not less than 30 K. More specifically, the oxidesuperconductor thin film is formed of a high-To copper-oxide type oxidesuperconductor material typified by a Y-Ba-Cu-O type compound oxidesuperconductor material, a Bi-Sr-Ca-Cu-O type compound oxidesuperconductor material, and a TI-Ba-Ca-Cu-O type compound oxidesuperconductor material.

In addition, the substrate, on which the oxide superconductor thin filmis deposited, can be formed of an insulating substrate, preferably anoxide single crystalline substrate such as MgO, SrTiO₃, and CdNdAlO₄.These substrate materials are very effective in forming or growing acrystalline film having a high orientation property. However, thesuperconducting device can be formed on a semiconductor substrate if anappropriate buffer layer is deposited thereon. For example, the bufferlayer on the semiconductor substrate can be formed of a double-layercoating formed of a MgAl₂ O₄ layer and a BaTiO₃ layer, or a single layerof "YSZ" (yttrium stabilized zirconia) or Y₂ O₃ if a silicon substrateis used.

The above and other objects, features and advantages of the presentinvention will be apparent from the following description of preferredembodiments of the invention with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are diagrammatic sectional views of embodiments of thesuperconducting device in accordance with the present invention;

FIG. 2A to 2G are diagrammatic sectional views illustrating anembodiment of the process for manufacturing the superconducting devicesshown in FIG. 1A and 1B;

FIG. 3A to 3K are diagrammatic sectional views illustrating anembodiment of the process for manufacturing a second embodiment of thesuperconducting device in accordance with the present invention; and

FIG. 4A to 4L are diagrammatic sectional views illustrating anembodiment of the process for manufacturing a third embodiment of thesuperconducting device in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment 1

Referring to FIGS. 1A and 1B, there are shown diagrammatic sectionalviews of two embodiments of the superconducting devices in accordancewith the present invention. In these Figures, elements similar to eachother are given the same Reference Numerals.

The superconducting device shown in FIG. 1 includes an oxidesuperconductor thin film 1 formed on a substrate 5 and having anon-superconducting region 50 which is formed by diffusion ofconstituent element(s) into the oxide superconductor thin film and whichno longer has superconductivity. The oxide superconductor thin filmforms a superconducting channel 10 of about five nanometers in thicknessat a position above the non-superconducting region 50. On thesuperconducting channel 10, an extremely thin gate electrode 4 isprovided through a gate insulator layer 6. A source electrode 2 and adrain electrode 3 are located on the oxide superconductor thin film 1 atopposite sides of the superconducting channel 10, respectively.

The gate electrode 4 is formed of a normal conductor thin film or anoxide superconductor thin film, and is deposited on a side surface of asurface protection layer 8 formed on the gate insulator 6. The normalconductor thin film can be deposited on the side surface of the surfaceprotection layer 8 by for example an inclined evaporation process. Theoxide superconductor thin film can be deposited on the side surface ofthe surface protection layer 8 by for example an off-axis sputtering. Asseen from the drawing, the thin film of the gate electrode 4 standsupright on the gate insulator, and therefore, the thickness of the thinfilm of the gate electrode 4 determines a gate length.

The embodiment shown in FIG. 1B is different from the embodiment shownin FIG. 1A, only in that the non-superconducting region 50 isconstituted of a projection previously formed on the substrate 5. Theother features of the embodiment shown in FIG. 1B are completely thesame as those of the embodiment shown in FIG. 1A, and therefore, furtherexplanation will be omitted.

Now, a process for manufacturing the superconducting device shown inFIG. 1A will be described with reference to FIGS. 2A to 2G.

First, a substrate 5 is prepared as shown in FIG. 2A. This substrate 5is formed of for example, an insulating substrate such as a MgO (100)substrate, a SrTiO₃ (100) substrate or others, or a semiconductorsubstrate such as a silicon (100) substrate having a principal surfacecoated with insulating films. However, if the silicon substrate is used,a principal surface of the silicon substrate is continuously coated withMgAl₂ O₄ by a CVD process and with BaTiO₃ by a sputtering process.

Then, as shown in FIG. 2B, an oxide superconductor thin film 1 having athickness on the order of 200 nm to 300 nm is deposited on the substrate5, by for example an off-axis sputtering, a reactive evaporation, an MBE(molecular beam epitaxy), a CVD, etc. The oxide superconductor materialis preferably formed of, for example, a Y-Ba-Cu-O type compound oxidesuperconductor material, a Bi-Sr-Ca-Cu-O type compound oxidesuperconductor material, and a Tl-Ba-Ca-Cu-O type compound oxidesuperconductor material. The oxide superconductor thin film ispreferably formed of a c-axis orientated oxide superconductor thin film,since the c-axis orientated oxide superconductor thin film has a largecritical current density in a direction parallel to the substrate. Thec-axis orientated oxide superconductor thin film can be formed bymaintaining the substrate temperature at about 700° C. in a filmdeposition process.

Next, a laser beam, or a focused ion beam is locally irradiated onto theoxide superconductor thin film 1, as shown by arrows in FIG. 2C, so thatconstituent element(s) of the substrate 5 is caused to diffuse into theoxide superconductor thin film 1 so as to form the non-superconductingregion 50. A portion of the oxide superconductor thin film remainingabove the non-superconducting region 50 forms the superconductingchannel 10.

In the case of forming the non-superconducting region 50 by irradiatingthe laser beam, it is preferable to use a high power laser beam such asan excimer laser, a CO₂ laser, a YAG laser,, etc. For example, when anAr laser having a wavelength of 514 nm is used, it is preferred to scanthe laser beam having an irradiation output power of 2.0 W at a speed of100 μm/second, On the other hand, in the case of forming thenon-superconducting region 50 by irradiating the focused ion beam, it ispreferred to irradiate Ar ions with a beam diameter of not greater than0.2 μm and an acceleration voltage of not greater than 50 KV.

Here, the process shown in FIGS. 2B and 2C can be replaced by a processillustrated in FIGS. 2BB and 2CC.

Namely, as shown in FIG. 2BB, an focused ion beam is locally irradiatedonto the substrate 5 so as to form a doped region 51. The irradiatedions preferably are Ba ions, Y ions, or Cu ions. In addition, it is alsopreferred that the beam diameter is 0.2 μm and the acceleration voltageis 50KV. The doped region 51 having a width of not greater than 1 μm isformed on the principal surface of the substrate 5 by irradiation of thefocused ion beam.

Thereafter, as shown in FIG. 2CC, the oxide superconductor thin film 1is deposited on the substrate 5 having the doped region 51, by forexample the off-axis sputtering, the reactive evaporation, the MBE, theCVD, etc., similarly to the process of FIG. 2C. In the process of growthof the oxide superconductor thin film 1 on the substrate 5, dopedelements are diffused from the doped region. 51 into the oxidesuperconductor thin film 1 so as to form the non-superconducting region50. A superconducting portion of the oxide superconductor thin filmremaining above the non-superconducting region 50 forms thesuperconducting channel 10.

Thereafter, as shown in FIG. 2D, an insulator film 16 is formed on theoxide superconductor thin film 1, and then, surface protectioninsulating layers 8 and 9 are formed on the insulator film 1. 6,excluding a limited region above the superconducting channel 10. Theinsulating layer 16 is preferably formed of an insulating material suchas MgO, which does not form a large density of energy levels between thesuperconductor thin film 1 and the insulating layer 16. In addition, theinsulating layer 16 has a thickness sufficient to prevent a tunnelcurrent, for example, a thickness of not less than 10 nanometers. Thesurface protection insulating layers 8 and 9 are preferably formed ofMgO.

As shown in FIG. 2E, a normal conductor film 18 is formed on the surfaceprotection layer 8 by performing an evaporation in an inclined directionso as to ensure that the normal conductor film 18 is deposited on a sidesurface 8A of the surface protection layer 8 positioned above thesuperconducting channel 10. In this process, another normal conductorfilm 19 is simultaneously formed on the surface protection layer 9.However, this normal conductor film 19 is not necessary. These normalconductor films 18 and 19 are preferably formed of a refractory metalsuch as Ti, W, etc., or Au, or a silicide thereof.

Then, as shown in FIG. 2F, an anisotropic etching is conducted on thenormal conductor films 18 and 19 by means of a reactive ion etching, oran Ar-ion milling, so that a portion of the normal conductor film 18remains only on the side surface 8A of of the surface protection layer8. This remaining normal conductor film 18 forms a gate electrode 4.This gate electrode 4 is preferred to have a thickness of not greaterthan 100 nm.

Finally, as shown in FIG. 2G, the insulator film 16 and the surfaceprotection layers 8 and 9 are removed from opposite end regions of theoxide superconductor thin film 1. Therefore, the insulator film 16remaining above the superconducting channel 10 forms a gate insulatorlayer 6. On the other hand, a source electrode 2 and a drain electrode 3are formed on the opposite exposed end regions of the oxidesuperconductor thin film 1. The source electrode 2 and the drainelectrode 3 are formed of the same normal conductor material as that ofthe gate electrode. Thus, the superconducting channel-FET is completed.

In the above mentioned embodiment, the non-superconducting region 250 isformed by diffusing the constituent element(s) of the substrate into theoxide superconductor thin film 1. The present invention is not limitedto this method. For example, in the case of manufacturing thesuperconducting device as shown in FIG. 1B, the substrate is previouslymachined or etched so as to have a projection corresponding to thenon-superconducting region 50, and then, the oxide superconductor thinfilm I is deposited on the substrate surface having the projection, andthereafter, is planarized so that the oxide superconductor thin film Ihas an flat upper surface.

In the above mentioned embodiment, the gate electrode is formed of anormal conductor, but can be formed of an oxide superconductor. In thelatter case, after the insular film 16 is formed, an oxidesuperconductor thin film which has a thickness of not greater than100rim and which is preferably an a-axis oriented film, is deposited,and then, the Ar-ion milling and the anisotropic etching are performedin an inclined direction so as to shape a superconducting gateelectrode. Thereafter, the protection layers 8 and 9 are formed.

Embodiment 2

Referring to FIG. 3K, there are shown diagrammatic sectional views of asecond embodiment of the superconducting device in accordance with thepresent invention. In FIG. 3K, elements similar to those shown in FIGS.1A to 2G are given the same Reference Numerals.

The superconducting device shown in FIG. 3K includes a gate insulatorlayer 6 and an oxide superconductor thin film 1 formed in the namedorder on a substrate 5 having a superconducting gate electrode 4embedded therein. A surface protection layer 7 is formed on a portion ofthe oxide superconductor thin film 1 above the superconducting gateelectrode 4. A source electrode 2 and a drain electrode 3 are formed onthe oxide superconductor thin film 1 at both sides of the surfaceprotection layer 7, respectively.

The oxide superconductor thin film 1 is formed of a c-axis orientedoxide superconductor crystal layer having a thickness of not greaterthan about five nanometers, so that a portion of the oxidesuperconductor thin film 1 above the embedded superconducting gateelectrode 4 forms the superconducting channel 10. The gate insulatorlayer 6 is preferably formed of an insulating material such as MgO, Si₃N₄, and has a thickness sufficient to prevent a tunnel current, forexample, a thickness of not less than 10 nanometers. The superconductinggate electrode 4 is formed of an a-axis oriented oxide superconductorcrystal layer and has a thickness of not greater than about 100 nm in adirection of an electric current flowing through the superconductingchannel 10.

Now, a process for manufacturing the superconducting device shown inFIG. 3K will be described with reference to FIGS. 3A to 3K.

First, a substrate 5 is prepared as shown in FIG. 3A. Similarly to thefirst embodiment, this substrate 5 is formed of for example, aninsulating substrate such as a MgO (100) substrate, a SrTiO₃ (100)substrate or others, or a semiconductor substrate such as a silicon(100) substrate having a principal surface coated with an insulatingfilm. However, if the silicon substrate is used, the silicon substrateis coated with an insulating film after a step explained hereinafter isformed.

As shown in FIG. 3B, a photoresist 20 is deposited and patterned tocover a portion 5A of the substrate, and then, an uncovered portion ofthe substrate 5 is etched by a dry etching such as a reactive ionetching and an Ar-ion milling, so that a step 53 is formed as shown inFIG. 3C. Thereafter, the photoresist 20 is removed.

In the case that a semiconductor substrate is used, a crystallinedirection is important, and therefore, the process is modified. Forexample, if a silicon substrate is used, a photoresist mask 20 is formedso as to ensure that a gate length direction (a channel currentdirection) is perpendicular to a Si(110) plane. The silicon substratepartially masked with the photoresist 20 is etched with an etchingliquid such as KOH or APW, so that a step 53 is formed as shown in FIG.3C. After the photoresist mask 20 is removed, the principal surfacehaving the step 53 is continuously coated with MgAlO₄ by a CVD (chemicalvapor deposition) and with BaTiO₃ by a sputtering process.

Then, as shown in FIG. 3D, an a-axis oxide superconductor thin film 11having a thickness of not greater than 100 nm is deposited on theprincipal surface of the substrate 5 at a substrate temperature of notgreater than 650° C., by for example an off-axis sputtering, a reactiveevaporation, an MBE (molecular beam epitaxy), a CVD, etc. The oxidesuperconductor material is preferably formed of, for example, aY-Ba-Cu-O type compound oxide superconductor material, a Bi-Sr-Ca-Cu-Otype compound oxide superconductor material, and a TI-Ba-Ca-Cu-O typecompound oxide superconductor material. The a-axis orientated thin filmhas a large critical current density in a direction perpendicular to thesubstrate surface.

The oxide superconductor thin film 11 is selectively removed from aprojected portion 5A and a recessed portion 5B of the substrate surfaceby an anisotropic etching such as a reactive ion etching, so that asuperconducting gate electrode 4 is formed on only a side surface of thestep 53 as shown in FIG. 3E.

The same material as that of the substrate 5 is deposited by sputteringso that a layer 54 having a thickness sufficient to perfectly fill therecessed portion 5B, as shown in FIG. 3F. In addition, a photoresist(not shown) is deposited to cover the layer 54 and to have a fiat uppersurface. Thereafter, as shown in FIG. 3G, the photoresist on the layer50 and the layer 50 itself are etched backed and planarized by theAr-ion etching until an upper end of the superconducting gate electrode4 is exposed.

As shown in FIG. 3H, a gate insulator layer 6 is formed on theplanarized surface of the substrate 5. This gate insulator layer 6 ispreferably formed of an insulating material such as MgO, which does notform a large density of energy levels between the oxide superconductorthin film and the gate insulator layer 6, In addition, the gateinsulator layer 6 has a thickness sufficient to prevent a tunnelcurrent, for example, a thickness of not less than 10 nanometers.

As shown in FIG. 31, a c-axis oxide superconductor thin film 1 having athickness of not greater than five nanometers is deposited on the gateinsulator layer 6. This c-axis oxide superconductor thin film 1 can beformed at a substrate temperature of about 700° C., by for example anoff-axis sputtering, a reactive evaporation, the MBE, the CVD, etc. Thec-axis orientated thin film has a large critical current density in adirection in parallel to the substrate surface.

As shown in FIG. 3J, a surface protection layer 17 is deposited to coverthe whole of the oxide superconductor thin film 1. Thereafter, thesurface protection layer 17 is selectively removed so that the surfaceprotection layer 17 remains only above the superconducting gateelectrode 4. A source electrode 2 and a drain electrode 3 are formed onthe oxide superconductor thin film 1 at both sides of the remainingsurface protection layer 7. The source electrode 2 and the drainelectrode 3 are formed of a normal conductor, for example, a refractorymetal such as Ti, W, etc., or Au, or a silicide thereof, or an oxidesuperconducting material.

Embodiment 3

Referring to FIG. 4L, there are shown diagrammatic sectional views of athird embodiment of the superconducting device in accordance with thepresent invention. In FIG. 4L, elements similar to those shown in FIGS.1A to 3K are given the same Reference Numerals.

The superconducting device shown in FIG. 4L includes an oxidesuperconductor thin film 1 deposited on a substrate 5 so as to form asuperconducting channel. An insulating layer 61 is formed on the oxidesuperconductor thin film 1 above the superconducting gate electrode 4. Asuperconducting source electrode 2 and a superconducting drain electrode3 are formed on both end portions of the insulating layer 61,respectively. On a central portion of the insulating layer 61, there islocated a superconducting gate electrode 4 surrounded by an insulatinglayer 77.

The substrate 5 is formed of for example, an insulating substrate suchas a MgO (100) substrate, a SrTiO₃ (100) substrate, a CdNdAlO₄ (100)substrate or others. The oxide superconductor thin film 1 is formed of ac-axis oriented oxide superconductor crystal layer having a thickness ofnot greater than about five nanometers, and on the other hand, thesuperconducting source electrode 2 and the superconducting drainelectrode 3 are formed of an a-axis oriented oxide superconductorcrystal layer having a thickness of about 200 nm. The superconductinggate electrode 4 is similarly formed of an a-axis oriented oxidesuperconductor crystal layer and has a thickness of not greater thanabout 100 nm in a direction in parallel to the oxide superconductor thinfilm 1. The insulating layer 77 surrounding the superconducting gateelectrode 4 is preferably formed of an insulating material such as MgO,Si₃ N₄, and has a thickness sufficient to prevent a tunnel current, forexample, a thickness of not less than 10 nanometers.

The insulating layer 61 is formed of a MgO film having a thickness ofnot greater than 10 nanometers. In a portion of the insulating layer 61under each of the superconducting source electrode 2 and thesuperconducting drain electrode 3, MgO flocculates so that the oxidesuperconductor thin film 1 is in electric connection with thesuperconducting source electrode 2 and the superconducting drainelectrode 3, respectively.

Now, a process for manufacturing the superconducting device shown inFIG. 4L will be described with reference to FIGS. 4A to 4L.

First, a substrate 5 is prepared as shown in FIG. 4A. Similarly to thefirst embodiment, this substrate 5 is formed of for example, aninsulating substrate such as a MgO (100) substrate, a SrTiO₃ (100)substrate or others, or a semiconductor substrate such as a silicon(100) substrate having a principal surface coated with an insulatingfilm. For example, if the silicon substrate is used, a principal surfaceof the silicon substrate is continuously coated with MgAl₂ O₄ by a CVDprocess and with BaTiO₃ by a sputtering process.

Then, as shown in FIG. 4B, a c-axis oxide superconductor thin film 1having a thickness of not greater than five nanometers is deposited onthe substrate 5. This c-axis oxide superconductor thin film 1 can beformed by for example an off-axis sputtering, a reactive evaporation,the MBE, the CVD, etc. The c-axis orientated thin film has a largecritical current density in a direction in parallel to the substratesurface.

For example, the oxide superconductor thin film I can be formed by anoff-axis sputtering which is performed under the condition that asputtering gas is composed of Ar and O₂ at the ratio of Ar: O₂ =9:1, thesputtering gas pressure is 10 Pa, and the substrate temperature is 700°C.

Then, an insulating layer 61 is formed on the oxide superconductor thinfilm 1, as shown in FIG. 4C. The thickness of the insulating layer 61 ismade to be not greater than ten nanometers. The insulating layer 61 isformed of an insulating material such as MgO, which can realize anelectric connection due to flocculation.

As shown in FIG. 4D, a sublimation type resist layer 80 of for exampleMo is formed on a right half (in the drawing) of the insulating layer61, by a vacuum evaporation process.

As shown in FIG. 4E, an a-axis oxide superconductor thin film having athickness of about 200 nm is deposited on a portion of the insulatinglayer 61 which is not covered by the resist layer 80, so that asuperconducting source electrode 2 is formed. This a-axis oxidesuperconductor thin film can be formed by for example an off-axissputtering, a reactive evaporation, the MBE, the CVD, etc., and at asubstrate temperature of not greater than 650° C.

For example, the oxide superconductor thin film for the superconductingsource electrode 2 can be formed by an off-axis sputtering which isperformed under the condition that a sputtering gas is composed of Arand O₂ at the ratio of Ar: O₂ =9:1, the sputtering gas pressure is 10Pa, and the substrate temperature is 640° C.

In a portion of the insulating layer 61 under the superconducting sourceelectrode 2, MgO flocculates, so that an electrical connection is formedbetween the oxide superconductor thin film 1 and the superconductingsource electrode 2. In the deposition process of the superconductingsource electrode 2, on the other hand, the sublimation type resist layer80 sublimates, so that the fight half of the insulating layer 61 isexposed.

As shown in FIG. 4F, an insulating layer 70 such as MgO or Si₃ N₄ isdeposited to continuously and uniformly cover the superconducting sourceelectrode 2 and the insulating layer 61. The insulating layer 70cooperates with the insulating layer 61 so as to form a gate insulatorlayer, and therefore, the insulating layer 70 is made to have athickness of not greater than ten nanometers in order to prevent atunnel current.

Thereafter, as shown in FIG. 4G, an oxide superconductor thin film 14 isdeposited on the insulating layer 70. This oxide superconductor thinfilm 14 is formed of an a-axis oriented oxide superconductor crystallayer having a thickness of not greater than about 100 nm. This oxidesuperconductor thin film 14 can be formed by an off-axis sputteringsimilarly to the superconducting source electrode 2 by maintaining thesubstrate temperature not greater than 650° C.

As shown in FIG. 4H, the oxide superconductor thin film 14 and theinsulating layer 70 are anisotropically etched by a reactive ionetching, an Ar-ion milling or other suitable means, so that the oxidesuperconductor thin film 14 and the insulating layer 70 are left on onlya side surface of the superconducting source electrode 2. The remainingsuperconductor thin film 14 forms a superconducting gate electrode 4. Onthe other hand, an etching is performed so that the right half portionof the insulating layer 61 is exposed again.

As shown in FIG. 4I, an insulating layer 72 is deposited to continuouslyand uniformly cover the superconducting source electrode 2, theremaining insulating layer 70, the superconducting gate electrode 4 andan exposed surface of the insulating layer 61. The insulating layer 72is formed of the same material as that of the insulating layer 70.

As shown in FIG. 4J, the insulating layer 70 is anisotropically etchedby a reactive ion etching, an Ar-ion milling or Other suitable means, soas to finish the superconducting gate electrode 4 surrounded by theinsulating layer 77. An etching is performed so that the fight halfportion of the insulating layer 61 is exposed again.

As shown in FIG. 4K, an oxide superconductor thin film 13 is depositedto continuously and uniformly cover the superconducting source electrode2, the insulating layer 77, the superconducting gate electrode 4 and anexposed surface of the insulating layer 61. This oxide superconductorthin film 13 is formed of an a-axis oriented oxide superconductorcrystal layer having a thickness of not greater than about 200 nm. Thisoxide superconductor thin film 13 can be formed by an off-axissputtering similarly to the superconducting gate electrode 4 bymaintaining the substrate temperature not greater than 650° C. At aboundary between the insulating layer 61 and the oxide superconductorthin film 13, MgO flocculates, so that an electrical connection isformed between the oxide superconductor thin film I and the oxidesuperconductor thin film 13.

Thereafter, a photoresist is deposited to cover the oxide superconductorthin film 13 and to have a flat upper surface, and etched back andplanarized until the upper end surface of the superconducting gateelectrode 4 is exposed as shown in FIG. 4L.

As explained above, in the superconducting device in accordance with thepresent invention, a main current flows through the superconductingchannel and is controlled by the gate voltage. Therefore, differentlyfrom the conventional super-FET in which a superconducting current flowsthrough the semiconductor channel due to the superconducting proximityeffect, the fine processing techniques which had been required formanufacturing the super-FET have become unnecessary. In addition, sinceit is not necessary to stack the superconductor and the semiconductor,high performance superconducting device can be realized by using anoxide superconductor. Furthermore, since the gate length can beshortened, the superconducting device in accordance with the presentinvention can operate at a high speed. Therefore, the application of thesuperconduction technology to the electronic devices can be promoted.

The invention has thus been shown and described with reference to thespecific embodiments. However, it should be noted that the presentinvention is in no way limited to the details of the illustratedstructures but changes and modifications may be made within the scope ofthe appended claims.

We claim:
 1. A superconducting device comprising:a substrate, an oxidesuperconductor thin film deposited on a deposition surface of saidsubstrate, said superconductor thin film defining a superconductingchannel therein; a source electrode and a drain electrode formed on saidoxide superconductor thin film at opposite ends of said superconductingchannel, so that a superconducting current can flow through saidsuperconducting channel between said source electrode and said drainelectrode, and a gate electrode formed on a gate insulator layer on saidsuperconducting channel so as to control superconducting current flowthrough said superconducting channel, said gate electrode being providedby a thin film which has been formed perpendicular with respect to saidgate insulator layer whereby, in a direction of current flow throughsaid superconducting channel between said source and drain electrodes,said gate electrode having a gate length of no more than 100 nm.
 2. Asuperconducting device claimed in claim 1 wherein said insulator layeris formed on said superconducting channel above said substrate and saidgate electrode in the form of a thin film is formed on said insulatorlayer above said substrate.
 3. A superconducting device claimed in claim2 wherein said superconducting channel has a thickness of not greaterthan five nanometers, and said oxide superconductor thin film of saidsuperconducting channel has an upper planar surface, wherein said gateinsulator layer is deposited on said upper planar surface of said oxidesuperconductor thin film, and an insulating protection layer is formedon the gate insulator layer and having an end surface angled to saidgate insulator layer, wherein said gate electrode is formed of a thinfilm of a normal conductor deposited on said end surface of saidinsulating protection layer.
 4. A superconducting device claimed inclaim 2 wherein said source electrode and said drain electrode areconstituted of a superconducting source electrode and a superconductingdrain electrode, respectively, which are located on an insulator layerwhich is on the oxide superconductor thin film of the superconductingchannel, and wherein said superconducting source electrode and saidsuperconducting drain electrode are in electrical connection with saidoxide superconductor thin film of said superconducting channel becauseof flocculation in the insulator layer sandwiched between said oxidesuperconductor thin film of said superconducting channel and each ofsaid oxide superconducting source electrode and said oxidesuperconducting drain electrode.
 5. A superconducting device claimed inclaim 4 wherein said insulator layer is formed of a MgO film having athickness of not greater than ten nanometers.
 6. A superconductingdevice claimed in claim 5 wherein said oxide superconductor thin film ofsaid superconducting channel is formed of an oxide superconductorcrystal layer having a thickness, in a direction perpendicular to adirection of electric current flow through said channel, of not greaterthan five nanometers and a c-axis orientation with respect to saidsubstrate, and each of said superconducting source electrode, said gateelectrode and said superconducting drain electrode is formed of an oxidesuperconductor crystal layer which has an a-axis orientation withrespect to said substrate.
 7. A superconducting device claimed in claim1 wherein a protection layer is formed on said gate insulator layer tohave an end surface above said superconducting channel, said end surfaceof said protection layer being substantially perpendicular to saidsubstrate, and said gate electrode in the form of a thin film is formedof a conducting thin film which is deposited on said end surface of saidprotection layer and which is at only its lower end in contact with saidgate insulator layer.
 8. A superconducting device claimed in claim 7wherein said oxide superconductor thin film of said superconductingchannel is formed of an oxide superconductor crystal layer having athickness of not greater than five nanometers and a c-axis orientationwith respect to said substrate, and each of said source electrode, saidgate electrode and said drain electrode is constituted of asuperconducting electrode formed of an oxide superconductor crystallayer which has an a-axis orientation with respect to said substrate. 9.A superconducting device claimed in claim 1 wherein said oxidesuperconductor thin film is a high-Tc oxide superconductor material. 10.A superconducting device claimed in claim 9 wherein said oxidesuperconductor thin film is a high-Tc copper-oxide type oxidesuperconductor material.
 11. A superconducting device claimed in claim 9wherein said oxide superconductor thin film is formed of the samematerial selected from the group consisting of a Y-Ba-Cu-O type compoundoxide superconductor material, a Bi-Sr-Ca-Cu-O type compound oxidesuperconductor material, and a Tl-Ba-Ca-Cu-O type compound oxidesuperconductor material.
 12. A superconducting device claimed in claim 1wherein the substrate is formed of a material selected from the groupconsisting of a MgO (100) substrate, tt SrTiO₃ (100) substrate and aCdNdAlO₄ (001) substrate, and a semiconductor substrate.
 13. Asuperconducting device comprising:a substrate; a gate electrode embeddedin said substrate, a gate insulator deposited on a deposition surface ofsaid substrate, an oxide superconductor thin film deposited on said gateinsulator, said superconductor thin film defining a superconductingchannel therein; a source electrode and a drain electrode formed on saidoxide superconductor thin film at opposite ends of said superconductingchannel, so that a superconducting current can flow through saidsuperconducting channel between said source electrode and said drainelectrode, said gate electrode being provided by a thin film which hasbeen formed perpendicular with respect to said gate insulator layerwhereby, in a direction of current flow through said superconductingchannel between said source and drain electrodes, said gate electrodehaving a gate length of no more than 100 nm.
 14. A superconductingdevice claimed in claim 13 wherein said oxide superconductor thin filmof said superconducting channel is formed of an oxide superconductorcrystal layer having a thickness of not greater than five nanometers anda c-axis orientation with respect to said substrate, and each of saidsource electrode, said gate electrode and said drain electrode areconstituted of a superconducting electrode formed of an oxidesuperconductor crystal layer which has an a-axis with respect to saidsubstrate.